Concepts of Physics

IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

Variation of intensity of laser in water as seen from above and from a side


Variation of intensity of laser in water as seen from above and from a side.


If a laser beam is going in water (having some scattering centers such as soap particles) we can see it. This is due to scattering of light. But scattering intensity in a particular direction depends on the direction of polarization ( E-field). This experiment illustrates this fact very nicely.


A cubic glass vessel, water, dettol, dropper, laser in a stand.


  1. Put some water (say half the height of the vessel) in the vessel and mix few drops of dettol.
  2. Put on the laser torch and let the light beam pass through the water horizontally.
  3. Place two observers A and B, one looking at the beam from above and the other through a side surface.
  4. Now slowly rotate the laser torch in its stand about its own axis so that the beam direction is not disturbed. Ask observer A when does he/she finds the beam most intense. Ask B, if he/she also finds it most intense in this situation. Write your conclusions.
  5. Now send the beam at about 45 degree to the water surface. What changes occur when you rotate the laser torch?


When light beam is scattered from suspended particles, the direction of E-field of the incident beam decides the direction in which most of the scattered light will go.

If light is going in x-direction and the E-field is in z-direction, the atomic dipole moments of the scattering particles will be set in vibrations in z-direction. These will emit light (scattered light) preferentially in z-direction. So the eye in z-direction will see an intense beam whereas the eye in y-direction will not see it.


Figure to be added